1. Field of the Invention
This invention relates to magnetic cores for inductive devices and particularly to laminated magnetic cores for dynamoelectric machines and methods of making the same.
2. Description of the Prior Art
It is well known that dynamoelectric machines such as alternating current induction motors generally utilize an annular stator core within which a rotor member is concentrically disposed. The stator core is usually formed of a stack of thin laminations having suitable slots and teeth which define winding receiving openings and a circular bore for receiving the rotor member. Additionally, a number of through bolt holes are usually disposed at angularly spaced locations near the outer periphery of each lamination.
The laminations must be held in precise alignment with each other to provide a uniform air gap between the stator core and rotor member. A slight distortion of the air gap may cause a reduction in starting torque and output power, and a misaligned rotor may cause an increase in bearing wear. Misalignment and radial shifting may occur when the core is subjected to unbalanced torsional loading as the stator core is bolted down to a planar mounting surface. Therefore the laminations must be interconnected one to another to form a rigid assembly.
A widely practiced method of interconnecting the laminations to form a rigid, unitary core structure is known wherein a varnish or other suitable adhesive is interposed between laminations to prevent radial shifting. Generally, the assembled core is held in a suitable fixture and dipped in a varnish to allow a thin coating of varnish to be deposited between laminations as the varnish is drawn within by capillary action. After curing, the adhesive binds the laminations rigidly together, thus preventing relative movement.
In laminated core structures where adhesive bonding is utilized, problems have arisen in connection with thermoplastic flow of the adhesive when the core is heated to high temperatures during the curing process and also during thermal cycling incident to normal operation. Where thermoplastic flow has occurred within regions of high pressure concentration, e.g., in regions immediately surrounding through bolt holes, the low compressibility characteristic of the core is substantially degraded which results in a loss of torque retention. This situation is particularly serious in those installations where through bolts are employed to secure the core to a planar surface in a cantilever mounting arrangement and the stator is held in concentric relation with a rotor member solely by the through bolts. For example, in hermetically sealed refrigeration motor compressor units, the stator core is frequently mounted in a cantilever fashion by means of through bolts which hold the stator in axial compression with a planar mounting surface. A loss of torque retention causes shifting of the entire stator core with respect to the rotor member as the through bolts loosen. In addition to the usual degradation in performance caused by a non-uniform air gap as discussed above, the stator may slip enough to cause the rotor member to rub against the slot teeth, thus causing lock-up of the rotor.
Many attempts have been made heretofore to minimize shifting in adhesive impregnated cores. For example, it is known to use laminations having unusually rough surface texture in combination with a uniform distribution of adhesive throughout interlaminar regions. The unusually rough surfaces are said to provide metal-to-metal point distributed contact with the adhesive distributed therebetween. However, any advantage provided by this structure is often offset by the corresponding reduction in power output which necessarily results from a decrease in core density caused by the excessive surface roughness.
It is also known to use laminations having normal surface roughness in combination with an uneven distribution of adhesive between laminations. This structure requires that the pressure concentration points, e.g., the through bolt hole regions, to be held under greater compression than is applied during normal operation so that all interlaminar spaces are filled, but with a lesser amount of adhesive migrating into the regions immediately surrounding the pressure concentration points. Because of the differential in the amount of adhesive, the laminations become deformed and assume the same relative position after repeated applications of compressive and torsional forces. However, although the amount of adhesive in the pressure concentrations regions is reduced, it is not entirely eliminated, and thermoplastic flow may be great enough to cause the through bolts to loosen.